There are many industrial and scientific applications that require surfaces to be uniformly irradiated by ion beams. For example, modification of semiconductors such as silicon wafers is often implemented by ion implanters, in which a surface is uniformly irradiated by a beam of ions or molecules of a specific species and energy. Because the physical size of the wafer or substrate (e.g., about 5 inches in diameter or more) is larger than the cross-sectional area of the irradiating beam (e.g., about 2 inches in diameter or less), the required uniform irradiance is commonly achieved by scanning the beam across the wafer or scanning the wafer through the beam, or a combination of these techniques.
It is distinctly advantageous to have a high beam scan rate over the substrate for a number of reasons: the irradiance uniformity is more immune to changes in the ion beam flux; a higher wafer throughput is possible at low dose levels; and for high dose applications degradation from local surface charging, thermal pulsing, and local particle-induced phenomena such as sputtering and radiation damage are greatly reduced.
Scanning techniques based only upon reciprocating mechanical motion are very limited in speed. Motion of the wafer on an arc through the beam greatly improves the scan speed but requires many wafers or substrates to be simultaneously mounted on a rotating carousel in order to obtain efficient utilization of the beam.
In a common variation, a time varying electric field is used to scan the beam back and forth in one direction, while the wafer is reciprocated in another direction. In this hybrid-type of implanter the beam current and hence rate at which wafers can be processed is severely limited by the space-charge forces which act in the region of the time-varying electric deflection fields. These forces cause the ions in the beam to diverge outward, producing an unmanageably large beam envelope. Such a space-charge limitation also occurs in implanters that use time-varying electric fields to scan the beam in two directions.
Space-charge blow-up is the rate at which the transverse velocity of a beam increases with distance along the beam axis. This is proportional to a mass normalized beam perveance EQU .xi.=I M.sup.1/2 E.sup.-3/2 ( 1)
where I is the beam current, M is the ion mass, and E is the ion energy. (The Physics and Technology of Ion Sources, Ed. Ian G. Brown, John Wiley & Sons, New York 1989). For typical ion beam configurations encountered in ion beam implanters, space-charge effects become limiting at a perveance of .xi..congruent.0.02[mA][amu].sup.1/2 [keV].sup.-3/2. Thus, an 80 keV arsenic beam becomes space-charge limited at .congruent.1.7 mA, while a 5 keV beam is space-charge limited at just .congruent.0.03 mA. Therefore, scanning an ion beam with an oscillatory electric field is not viable for an efficient commercial ion implanter in which the beam current is preferably greater than a few milliamperes, even at energies as low as 10 keV.
A scanning magnet that produces a high frequency time-varying magnetic field for scanning ion beams in implanters is described by one of the inventors in U.S. Pat. No. 5,311,028, in which a scanning magnet that has a yoke formed from laminations of high magnetic permeability separated by relatively thin electrically insulating material can be used to scan high perveance, heavy ion beams at frequencies up to 1,000 Hz.